Vibration damping resins displaying viscoelastic behavior for use in forming metal laminates are known. For example, U.S. Pat. No. 4,859,523, the teachings of which are incorporated herein by reference, describes polyurethanes useful for forming metal-resin-metal composites. The viscoelastic resin layer, that adheres two metal layers, damps vibration by converting external vibrational energy to heat energy. Vibration damping is useful in reduction of noise and prevention of metal fatigue. Vibration-damped metal has a wide variety of applications where vibrational noise is of concern, particularly in the automotive industry. The use of vibration damping composites is known for oil pans, engine covers, rocker panels, air filters covers, and other automotive parts.
A great deal is expected of a resin whose intended use is to damp vibration, particularly when it is desirable to damp vibration over a wide temperature range. In a typical process for forming a vibration-damping composite, the resin can be applied to a metallic substrate by various techniques, typically by coil-line technology.
In forming a component part, the laminate is shaped by deep drawing and/or stamping. If the formed part is an automotive part, it will be part of the automobile and exposed to about 400.degree. F. for about 45-60 minutes to bake the paint coating on the car. In use, it is desirable for the composite to damp vibration over a wide operating temperature range (this range may vary widely, depending upon the use environment of the component).
It can be appreciated that a viscoelastic resin must have chemical and physical stability over a wide temperature range. It must also be able to both adhere the layers of metal together and effectively damp vibration over a wide temperature range. Throughout the entire processing temperature range of the laminate-forming process, component-forming process, and baking process, the resin must not ooze from between the metal layers. The resin should provide sufficient peel strength upon formation of the composite so as to survive passage through the coil coating/laminating process or any other conditions selected to form the composite. To withstand the drawing and/or stamping steps which occurs during component formation, high lap shear strength is required.
One of the specific goals for a resin in accordance with this invention is to obtain, over a broad operating temperature range, a composite loss factor or tan delta (tan D) of at least about 0.05 and preferably of at least about 0.1. Loss factor is a measure of conversion of external vibrational energy into heat energy by internal friction in the resin layer. The higher the loss factor, the greater the amount of vibrational energy that is converted to heat. This value may be measured on an Oberst-Beam by ASTM procedure E756-83. The goal of obtaining a high loss factor over a broad temperature range is desirably tied in to the ability of the resin to be used on a coil-line which has radical processing conditions involving mechanical stresses during the fabrication process and time/temperature parameters which can engender reaction kinetics completely unknown to anyone. A minimum shear strength of about 1000 psi at room temperature (e.g., 25.degree. C.) is sought. Additionally, decrease in lap shear must be minimal at elevated temperatures; the lap shear should be about 750 psi at 250.degree. F. A minimum peel strength of at least about 8, and preferably at least about 12 lbs/inch is sought for room temperature values. Furthermore, there should be no loss in damping or mechanical properties after a one-hour bake at 400.degree. F. when tested at room temperature.
Resins used in vibration-damping applications frequently contain nitrogen atoms in their structure, and this is undesirable when end-items containing such polymers are exposed to pyrolytic temperatures associated with welding operations. Under such conditions, the nitrogen will form toxic, gaseous decomposition products, such as hydrogen cyanide, which are particularly hazardous in the immediate work environment. Other elements, such as chlorine, and sulfur and phosphorus also produce hazardous gases during the decomposition that occurs at the weld site. It is an object of the present invention to provide vibration-damping resins which contain only the elements hydrogen, oxygen and carbon, and which, nevertheless, have all the above-mentioned desirable attributes for a vibration-damping resin.